Process for electrolytic recovery of metallic gallium

ABSTRACT

Gallium metal is electrolytically extracted from gallium compounds by a process which simultaneously dissolves the compound and reduces the metal.

United States Patent [191 Sleppy et all I Sept. 9, 1975 PROCESS FORELECTROLYTIC RECOVERY OF IVIETALLIC GALLIUM Inventors: William C.Sleppy, Belleville;

Robert H. Goheen, OFallon, both of Assignee: Aluminum Company ofAmerica,

Pittsburgh, Pa.

Filed: Oct. 16, 1974 Appl. No.: 515,250

US. Cl 204/105 R; 423/158; 423/602 Int. Cl. C25c 1/22 Field of Search204/105 R; 423/158, 602

[56] References Cited UNITED STATES PATENTS 3,325,383 6/1967lwantschefi" et a] 204/105 R 3,677,918 7/1972 Miyake 204/105 R PrimaryExaminerR. L. Andrews Attorney, Agent, or Firm.lohn P. Taylor, Esq.

[57] ABSTRACT Gallium metal is electrolytically extracted from galliumcompounds by a process which simultaneously dissolves the compound andreduces the metal.

9 Claims, 3 Drawing Figures PROCESS FOR ELECTROLYTIC RECOVERY OFMETALLIC GALLIUM BACKGROUND OF THE INVENTION This invention relates tothe electrolytic production of gallium metal. More particularly, theinvention relates to the electrolytic production of metallic galliumdirectly from a gallium compound which comprises one electrode of theelectrolytic cell. 7

The demand for gallium metal is rapidly expanding due, at least in part,to its use in the electronics industry in applications such as lightemitting diodes and microwave cavity devices. The manufacturingprocesses'for such devices generate much scrap in the form of galliumcompounds such as GaAs, GaP, and GaAsP.

The reclamation of metallic gallium from these compounds is complicatedby the insolubility of the compounds in conventional solvents. Becauseof this insolubility, gallium compounds are conventionally recycled torecover the gallium by processes which require a large amount of energyin the form of heat. For example, gallium may be recovered from galliumarsenide (GaAs) by volatilization of arsenide vapors as A However, thisrequires a temperature of at least 1,200C as well as a dry andoxygen-free system.

In an alternate system, the gallium compound is dryfused with a basesuch as NaOI-l at 450C to convert the gallium into sodium gallate whichcan then be dissolved in an aqueous basic solvent such as NaOH prior tosubsequent electrolytic reduction of the gallium metal.

In view of the rising costs of energy and the desire for conservation ofavailable energy, it would be desirable to eliminate the need forconversionvia heatof in soluble gallium compounds into more solubleforms.

SUMMARY OF THE INVENTION Quite surprisingly, it has now been discoveredthat gallium compounds may be dissolved electrolytically in the samecell used for reduction of gallium metal by making the solid galliumcompound the anode of the cell. In a preferred embodiment, theundissolved compound is present in particulate form and is contained inan electrolytically permeable holder which may also have conductivemeans therein relatively inert to the electrolytic process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic crosssectional view of a cell illustrating the invention. v Q

FIG. 2 is a cross sectional view of a multi-electrode bipolar cellconstructed in accordance with the invention.

FIG. 3 is a partially broken away end view of one of the bipolarelectrode assemblies shown in FIG. 2.

DETAILED DESCRIPTION a non-conductive hollow container 42 made of glassor plastic or the like in which the gallium compound may be placed inparticulate form as shown at 44.

Container 42 is provided with holes 46 and holes 48 to respectivelypermit entrance of electrolyte and exiting of generated gases. Filterpaper or cloth or other chemically inert permeable membrane 50 is placedover holes 46 to prevent loss of particulate gallium compound whilepermitting access to the gallium compound by the electrolyte.

Electrical contact with the gallium compound is maintained in theillustrated embodiment of FIG. 1 via a plate 52 which may be stainlesssteel or nickel or other chemically inert conductor such as graphite andwhich is biased against the gallium compound particles by a spring 54which also provides electrical contact to a terminal 56 which issuitably mounted on container 42.

A lead 58 connected to terminal 56 provides mechanical support for anodeassembly 40 as well as electrical connection to a suitable DC powersource illustrated in FIG. 1 as a battery 60.

In operation, a gallium compound such as GaAs is ground to a particlesize of about l0 to +48 mesh and placed in anode assembly 40 in cell 2.After electrical connection with DC power source 60, the anodicallypolarized GaAs dissolves into and reacts with the elec trolyte to formdissolved and hydrolyzed gallate and arsenite ions. Simultaneouselectrolysis of H 0 causing evolution of H at the cathode and 0 at theanode accompanies the anodic dissolution of the gallium compound. Thegallium containing ions, upon reaching cathode 30, are reduced togallium metal. Arsenite is built up in the bath as dissolved AsO whichgradually oxidizes to AsOf until the solubility of Na AsO .l 2- H O isexceeded.

To avoid precipitation of the hydrated sodium arsenate salt, theelectrolyte must be changed or treated in an appropriate manner toremove arsenate. In a preferred embodiment, electrolyte is continuouslypumped from the cell into a reaction vessel for treatment with lime toprecipitate an insoluble calcium arsenate as Ca (AsO The purified andregenerated electrolyte is then circulated back to the cell.

While the gallium compound has been illustrated in its preferred form asparticulate, a solid gallium compound anode could be used. However, theuse of the compound in particulate form increases the surface area thusincreasing dissolution and current efficiency with respect to cathodicdeposition of gallium. Furthermore, it has been found that, when solidanodes are used, minor amounts of the saltin undissolved formmay dustoff the anode and migrate to the cathode to, in effect, poison it. Thislatter effect can, of course, be minimized by the isolation of the anodefrom the cathode with filter cloth as previously described in connectionwith the description of anode assembly 40.

It has been further noted that the cathode should preferably beconditioned by an initial plating of gallium thereon. This can mostadvantageously be accomplished by adding a minor amount of dissolvedgallium to the electrolyte prior to start-up of the cell. This willpermit a sufficient initial deposition of gallium on the cathode priorto build-up of gallium ions in the bath due to dissolution of thegallium salt.

Turning now to FIG. 2, another embodiment is illustrated wherein theinvention is carried out using bipolar electrode assemblies. Anon-conductive housing comprises sidewalls 102 and 104, backwall 106, afront wall (not shown) and a bottom portion 110.

Within housing 100 and supported by bottom portion are a plurality ofbipolar electrode assemblies 120. Electrode assembly comprisesnon-conductive frame portions 122 and 124 between which is sandwiched afilter cloth 126 which together form one wall of the electrode assembly.A satisfactory filter cloth is No. 219 monofilament nylon weighing 150grams/m with about 50 filaments per cm. Other materials can besubstituted provided that they possess satisfactory chemical inertness,and are sufficiently fine so as to contain the particulate salt. Anothernon-conductive wall member 128 is spaced therefrom having mountedthereon a stainless steel electrode 130. A metallic stud member 132passes through member 128 and electrode 130 and carries a secondelectrode 134 which may be a metallic plate or screen which ispositioned parallel to and approximately midway between frame member 124and wall member 128.

Frame members 122 and 124 and wall member 128 extend from back wall 106of container 102 to the front wall to form a reservoir or chamber withstud member 132 protruding therein and positioning electrode 134 in theapproximate center of the chamber. The chamber is filled with a galliumcompound 44 such as GaAs in particulate form of preferably about 10 to+48 mesh. The gallium compound comprises the anode of each bipolarelectrode assembly while electrode 130 comprises the cathode. Electricalconnection therebetween is maintained via stud 132 and electrode 134.

An electrode 130b is mounted on wall 104 via a threaded bolt whichpasses through wall 104 and :provides an external terminal forconnection to the negative side of a DC source of power (not shown).This electrode comprises the cathode for the anodic portion of theadjacent bipolar electrode.

In similar fashion, electrode assembly 120a comprises only an anode withcontainer walls 102 forming one wall of the electrode assembly.Electrode 134a is centrally positioned within electrode assembly 1200 bya threaded bolt 142 which replaced stud 132 and which passes throughcontainer wall 102 to provide a positive terminal for connection to thepositive side of the DC source of power. An electrode 1300 is providednot to act as cathode in this case, but to provide total area of contactbetween the gallium compound and electrode 134a and 130a equivalent tothe contact area in the other bipolar electrodes.

After assembly of the electrodes, including placement of the galliumcompound therein, the container is filled with an electrolyte 144 whichis preferably a base such as NaOH at a concentration of about 100 to 200grams per liter (expressed as equivalent Na CO The electrolyte level issufficiently high to provide direct communication with the interior ofeach bipolar electrode through holes 136 in frame members 122 and 124and holes 138 in wall member 128. Holes 136 and 138 are respectivelypositioned to be above the level of gallium compound but below theelectrolyte level. This permits circulation of the dissolved salt tomaintain a constant concentration in the cell. The size of the holes isselected to allow only about 1% current leakage.

The bipolar cell is operated at a temperature above room temperature andpreferably at a temperature of about 70 to 90C to increase efficiency.This temperature is well above the melting point of metallic gallium andthus the gallium, as it deposits on the cathode 130, drops to the bottomof the cell. In this manner the gallium may be collected throughopenings provided in bottom portion 110 of the cell. Each opening isprovided with a valve 152 which controls the flow to a common collectiontrough 154.

As previously stated, the reduction of gallium is accompanied by anoxidation of the arsenic ion to an arsenite (AsO which gradually oxidesfurther to arsenate (AsOf). The arsenate ion content will graduallybuild up until the solubility of Na AsO is exceeded. While the elevatedoperating temperature permits accumulation of greater amounts ofarsenate, it eventually becomes necessary to remove some of the arsenateions. I

In the illustrated embodiment of FIG. 2 this is accomplished bycontinuously pumping a portion of electrolyte 144 into a reaction vesselvia pipes 154 and 156 and a pump 158. The arsenate-laden electrolyte istreated with an hydroxide such as calcium hydroxide or magnesiumhydroxide which will precipitate the arse nate. The precipitate (such ascalcium arsenate) settles in the reactor while the thus-purified andregenerated electrolyte is then returned to the cell via a pipe 162.

By providing appropriate hopper feed bins for each electrode assembly,it is possible to provide a continuously operating cell wherein galliumcompound is added to the electrodes and gallium metal and the anionicportion of the initial gallium compound are both continuously removed.

Each compartment optionally operates at about 4 to 5 volts with optimumcurrent densities of about 15 to 20 amps/clm on the cathode and 10 to 15amps/dm on the anode.

In actual use using 3 electrode assemblies, each having 9,450 cm volume,a total voltage of 24.8 and a total current of 133 Ampere hours, 0.94grams of gallium were produced per hour for a period of 18.8 hours.During this period of operation, periodic additions of gallium arsenidewere made to each electrode assembly and the spent electrolyte wascirculated through an external vessel for periodic treatment with lime.The current efficiency, i.e., that portion of the total current flowwhich resulted in reduction of gallium into metallic form, wascalculated to be 5.1% per electrode assembly or 15.3% for the entirecell.

Thus my invention provides for electrolytic dissolution of insolublegallium compounds while permitting simultaneous electrolytic reductionof gallium metal. 1n

the preferred embodiment, my invention may be prac ticed on a continuousbasis using bipolar electrodes.

While the invention has been described with respect to certainillustrated embodiments, it will be readily appreciated that minormodifications may be made while not departing from the spirit of theinvention.

What is claimed is:

1. A process for the recovery of gallium metal from gallium compoundswhich comprises:

a. establishing the gallium compound as a dissolvable anode in anelectrolytic cell;

b. passing a current between the gallium compound anode and an inertcathode to effect anodic dissolution of the gallium compound; and

c. simultaneously plating out metallic gallium on said cathode.

2. The process of claim 1 wherein the gallium compound comprises galliumarsenide and the amount of dissolved arsenate ion is controlled byremoval of a portion of said electrolyte and treating said electrolytewith a material which will cause precipitation of said arsenate.

3. The process of claim 2 wherein a portion of said electrolyte iscontinuously circulated from said cell into a reaction zone toprecipitate arsenate therefrom.

4. The process of claim 3 wherein said arsenatecontaining electrolyte istreated with lime to precipitate calcium arsenate from said electrolyte.

5. The process of claim 1 wherein said electrolytic cell contains anaqueous alkaline solution.

6. The process of claim 5 wherein said aqueous alkacell comprises aplurality of bipolar cells in series.

1. A PROCESS FOR THE RECOVERY OF GALLIUM METAL FROM GALLIUM COMPOUNDSWHICH COMPRISES: A. ESTABLISHING THE GALLIUM COMPOUND AS A DISSOLVABLEANODE IN AN ELECTROLYTIC CELL, B. PASSING A CURRENT BETWEEN THE GALLIUMCOMPOUND ANODE AND AN INERT CATHODE TO EFFECT ANODE DISSOLUTION OF THEGALLIUM COMPOUND, AND C. SIMULTANEOUSLY PLATING OUT METALLIC GALLIUM ONSAID CATHODE.
 2. The process of claim 1 wherein the gallium compoundcomprises gallium arsenide and the amount of dissolved arsenate ion iscontrolled by removal of a portion of said electrolyte and treating saidelectrolyte with a material which will cause precipitation of saidarsenate.
 3. The process of claim 2 wherein a portion of saidelectrolyte is continuously circulated from said cell into a reactionzone to precipitate arsenate therefrom.
 4. The process of claim 3wherein said arsenate-containing electrolyte is treated with lime toprecipitate calcium arsenate from said electrolyte.
 5. The process ofclaim 1 wherein said electrolytic cell contains an aqueous alkalinesolution.
 6. The process of claim 5 wherein said aqueous alkalinesolution contains at least 1 gram per liter of dissolved gallium priorto start-up of the cell to provide conditioning of the cathode byproviding deposition of a smooth clean liquid gallium coating on thecathode contemporaneous with the initial dissolution of the galliumcompound anode.
 7. The process of claim 1 wherein said gallium compoundanode is in particulate form.
 8. The process of claim 7 wherein saidparticulate gallium compound is contained in an electrolyticallypermeable holder.
 9. The process of claim 1 wherein said electrolyticcell comprises a plurality of bipolar cells in series.